The invention relates to syringes for injecting insulin and more specified pen-shaped syringes administering insulin doses from a cartridge in the pen syringe.
Diabetes is usually treated by the patient frequently injecting himself with an insulin dose which he adjusts each time according to his immediate need.
To make it less straining to the patient to prick himself several times a day, it is aspired to make the injections as painless as possible and to reduce the physical malaise many people will feel if they have to pass a needle into their own body.
As the malaise seems to grow with the length and the thickness of the needle and the sensation of pain seems to be reduced when the needle is made thinner, a passable way seems to be to make the needle thinner and shorter. This line may, of course, only be followed to a certain extent, as the needle must have a length permitting the subcutaneous injection of the insulin and a thickness allowing the insulin to pass through the needle.
Whereas the acceptable minimum length of the needle is well defined, the lower limit for the thickness of the needle is more fluid. With a thinner needle it becomes more difficult to press the insulin out through the needle and the injections will take more time. A more relevant lower limit is set by the fact that by injecting insulin types appearing as suspended crystals, a sieving of the suspension may occur, and the suspension injected may consequently have a lower concentration than expected.
The thickness of needles are indicated by a "G" and a gauge number increasing with thinner needles. Thus, the outer diameter of a G 27 needle is 0.4 mm, of a G 28 needle 0.36 mm, and of a G 30 needle 0.3 mm. The wall thickness of the needles is typically 0.075 mm, so that a G 27 needle has bore of 0.25 mm, whereas the bore of a G 30 needle is 0.15 mm.
Commonly, G 27 needles are used. However, according to Diabetes Forecast 1976; 29 page 27 problems are observed when G 27 needles and even thicker needles are used for injecting an insulin containing suspended crystals. The problem is a clogging of the needle during injection, which clogging is due to the fact that crystals of e.g. Lente insulin having a size of 20-40 .mu.m have a tendency to align themselves across the inside of the needle. This clogging is observed during injection, especially if this injection is carried out too slowly. It must be presumed that a similar clogging occurs during the filling of the syringe thus making the filling impossible or at least having the effect that some of the crystals are retained with the consequence that the suspension sucked into the syringe has a lower concentration then expected.
Whereas the trend goes towards the use of G 28 needles this is seen as close to the limit of what is possible. G 29 needles are seen as needles for disposable syringes for insulin, but G 30 needles have so far been deemed unusable for injection of an insulin suspension.